Field of the Invention
The present invention relates to a circuit board and an on-board structure of a semiconductor integrated circuit mounted on the circuit board, and particularly relates to a technique for surface-mounting a semiconductor integrated circuit to a circuit board.
Description of the Background Art
In recent years, semiconductor integrated circuits (ICs) are typically designed to dissipate heat generated by their power consumption through a heat spreader, namely, a heat-dissipating unit located in the rear surface of a package body. The heat spreader is attached or fixed to copper foil on a printed wiring board and dissipates heat generated by a semiconductor chip in the package toward the printed wiring board. Disclosed in Japanese Patent Application Laid-Open No. 2012-238670 is an on-board structure in which a semiconductor package with a heat spreader is bonded to a heat-dissipating land of a wiring board.
FIG. 5A is a plan view of a conventional printed wiring board, namely, a circuit board 50. FIG. 5B is a plan view of a semiconductor IC circuit 55 to be mounted on the circuit board 50. The semiconductor IC circuit 55 includes a package body 56 and leads, namely, terminals 57. In addition, as indicated by the broken line of FIG. 5B, a heat spreader, namely, a heat-dissipating unit 58 is located in the central part of the rear surface of the package body 56. As illustrated in FIG. 5A, the circuit board 50 includes both copper foil, namely, a heat-dissipating connection pad 51 disposed correspondingly to the heat-dissipating unit 58 and terminal connection pads 52 disposed correspondingly to the terminals 57.
To dissipate heat efficiently, the heat spreader needs to be securely bonded to the copper foil on the printed wiring board. FIGS. 6A and 6B each illustrate an on-board structure 60, in which the semiconductor IC circuit 55 is mounted on the circuit board 50. FIG. 6A is a plan view of the on-board structure 60. FIG. 6B is a cross-sectional view taken along the line E-E′ of FIG. 6A. The on-board structure 60 includes a bond 61 being solder between the heat-dissipating unit 58 and the heat-dissipating connection pad 51. FIG. 6B illustrates the on-board structure 60 obtained by mounting the semiconductor IC circuit 55 onto the circuit board 50 with the use of the optimal amount of bond 61. The heat-dissipating unit 58 is in direct contact with the bond 61. The bond 61 is in direct contact with the heat-dissipating connection pad 51.
FIG. 6B illustrates the on-board structure 60 obtained with the use of the optimal amount of bond 61. In actuality, however, it is difficult to optimally control the amount of bond 61 to be applied in mounting the semiconductor IC circuit 55 onto the circuit board 50. When the semiconductor IC circuit 55 is placed over the circuit board 50 on a trial basis prior to bonding in the mounting process, there is little clearance between the circuit board 50 and the semiconductor IC circuit 55, and thus, the junction therebetween is physically invisible. The amount of clearance between the circuit board 50 and the semiconductor IC circuit 55 varies with their product-to-product variations.
If the applied bond 61 falls short of the optimal amount, the bonding between the heat-dissipating unit 58 and the bond 61 or the bonding between the bond 61 and the heat-dissipating connection pad 51 would be poor. This can interfere with the thermal conduction from the heat-dissipating unit 58 to the heat-dissipating connection pad 51, thus causing the semiconductor chip to malfunction. Defective products including poorly-bonded components are likely to escape detection at inspections prior to shipment, and possibly get mixed with other products for sale on the market. Increasing the amount of bond 61 to be large enough to avoid the risk of poor bonding may be a workaround to abnormalities such as the poorly-bonded heat spreader. However, an excessive amount of bond 61 may cause the following problems.
The excess bond 61 extends off the heat-dissipating connection pad 51 and the heat-dissipating unit 58 and is pushed forward to the terminals 57 and therearound. The overflow of bond may cause troubles such as a short circuit between the terminals 57. The overflow of bond may be broken down into small particles, which can sometimes move freely in the space surrounded by a circuit board 10, the package body 56, and the terminals 57. Products in which small particles of bond have moved to places free from short circuits can escape detection at the lighting inspection of displays or the like prior to shipment. Thus, defective products can possibly get mixed with other products for sale on the market, as in the event of insufficient application of bond.